1. Field of the Invention
The present invention relates to a thin zoom optical system which is suited for use in an electronic image pickup apparatus such as a video camera, a digital camera or the like.
2. Description of the Prior Art
In recent years, digital cameras (electronic cameras) have been attracting attentions as cameras of the next generation which are to take place of silver salt 35 mm film cameras (Leica cameras by a popular name). These digital cameras have several broad categories ranging from high performance cameras for business purposes to portable popular type cameras.
Paying attention to a category of the portable popular type cameras out of these digital cameras, it is strongly demanded to obtain a video camera, a digital camera and the like which provide high quality images and have small depths.
A key point for configuring such a camera so as to have a small thickness in a direction of width lies in shortening a length of an optical system, a zoom optical system in particular, to be used in the camera as measured from a most object side surface to an image surface in a direction of depth.
A main stream has recently been formed by cameras adopting collapsible mount type lens barrels which move optical systems out of camera bodies at a photographing time and accommodate the optical systems into the camera bodies at a carriage time.
However, thicknesses of such cameras adopting the collapsible mount type lens barrels are largely different dependently on types of lens systems and filters used. In case of a zoom optical system in which a most object side lens unit has positive refractive power or the so-called positive lens unit leading type zoom optical system in particular, a thickness of each lens element and a dead space are enlarged in order to enhance specifications such as a zoom ratio, an F number and the like of the zoom optical system. It is impossible to configure a camera to be thin even when the camera uses the collapsible mount type lens barrel like a conventional example disclosed by Japanese Patent Kokai Publication No. Hei 11-258507. In contrast, a negative lens unit leading type zoom lens system which is composed of two lens units or three lens units in particular is advantageous for configuring a camera thin. A zoom lens disclosed by Japanese Patent Kokai Publication No. Hei 11-52246 is known as a conventional example of this type zoom lens system, but the zoom lens system uses lens units each composed of a large number of lens elements which are thick and when a leading lens element is a positive lens element, the zoom lens system does not allow a camera to be configured thin even if the camera is of the collapsible mount type.
Zoom optical systems disclosed by Japanese Patents Kokai Publication No. Hei 11-194274, No. Hei 11-287953 and No. 2000-9997 are known as conventional examples of zoom optical systems which are suited for use with currently known electronic image pickup devices, exhibit favorable imaging performance including zoom ratios, field angles, F numbers and the like, and have possibilities to permit thinning collapsed mounts.
Though it is desirable for thinning first lens units of these conventional zoom optical systems to thin locations of entrance pupils, but it is necessary for thinning the locations of the entrance pupils to enhance magnifications of second lens units. When the magnifications of the second lens units are enhanced, burdens are loaded on the second lens units, thereby making it difficult not only to thin the second lens units but also correct aberrations and undesirably enlarging influences due to manufacturing errors.
Furthermore, a camera can be configured thin and compact by configuring an image pickup device, but when a small image pickup device is to have an identical number of picture elements, the picture elements are arranged at a narrow pitch and a sensitivity becomes insufficient, whereby it is necessary to enhance the sensitivity with an optical system and an influence due to diffraction is produced.
An object of the present invention is to provide a zoom optical system comprising a first lens unit having negative refractive power, a second lens unit having positive refractive power and a third lens unit having positive refractive power, and configured to change a magnification from a wide position to a tele position by moving at least the first lens unit and the second lens unit so as to vary an airspace between the first lens unit and the second lens unit and widen an airspace between the second lens unit and the third lens unit, wherein the second lens unit comprises, in order from the object side, a biconvex lens element and a cemented meniscus lens component having a convex surface on the object side, and satisfies conditions (1) and (2) mentioned below.
Another object of the present invention is to provide a zoom optical system comprising a first lens unit having negative refractive power, a second lens unit having positive refractive power and a third lens unit having positive refractive power, and configured to change a magnification from a wide position to a tele position by moving at least the first lens unit and the second lens unit so as to vary an airspace between the first lens unit and the second lens unit and widen an airspace between the second lens unit and the third lens unit, wherein the second lens unit comprises, in order from the object side, of a biconvex lens element and a cemented meniscus lens component consisting of a positive meniscus lens element and a negative meniscus lens element, and having a convex surface on the object side, and satisfies the following conditions (1), (2), (3) and (4):
1.0 less than xe2x88x92xcex22T less than 2.2xe2x80x83xe2x80x83(1)
1.4 less than f2/fw less than 2.8xe2x80x83xe2x80x83(2)
0.05 less than D (2N)/D (2) less than 0.2xe2x80x83xe2x80x83(3)
0.2 less than R (2R)/f2 less than 0.5xe2x80x83xe2x80x83(4)